1. Field of the Invention
This invention relates to light metal pistons for internal combustion engines or compressors, that is, pistons made of aluminium, magnesium or similar metals and their alloys.
2. Description of the Prior Art
It has been proposed in my U.S. Pat. No. 3,908,521 to provide a light metal piston for an internal combustion engine or a compressor having a crown, a skirt and gudgeon pin bosses, and at least one insert made of a material of lower coefficient of thermal expansion than the light metal and shaped so that at least part of the insert conforms to the internal shape of the piston skirt, whereby to form a bi-metallic element with the light metal of the piston to effect deformation of the shape of the skirt when the piston is heated, each insert having at least one pair of bands extending at least part of the way around the interior of the piston from the regions of the gudgeon pin bosses, each band forming a bi-metallic element with the light metal, one band of said pair being spaced nearer to the piston crown than the other, and the bands of said pair being joined by a connecting portion of the insert in the region of the adjacent gudgeon pin boss.
In the above-mentioned U.S. Pat. in certain embodiments the upper and lower bands were disclosed as being of the same length, and in other embodiments either one or both the bands extended from one of the connecting portions on one side of the gudgeon pin boss around the interior of the piston to another of the connecting portions on the same side of the other gudgeon pin boss.
Both these arrangements have been found to have certain advantages. That in which the upper and lower bands are of the same length tends to effect greater control of thermal expansion near the open end of the skirt by the lower band than near the crown end of the skirt by the upper band, due to the greater stiffness of the skirt where it is joined to the crown. This results in the need to machine the skirt to a smaller diameter at the crown end than at the skirt end, whereas it is desirable that the skirt should be approximately cylindrical so that an approximately equal amount of metal is removed by machining over the whole length of the skirt.
In the embodiments in which a band extends around the interior of the piston from one gudgeon pin boss to the other, it is found that the continuous nature of the bands does not result, contrary to the expectation of those skilled in the art, in the desired degree of control of thermal expansion.